Optical system for head-mounted display

ABSTRACT

A head-mounted display may include a display system and an optical system that are supported by a housing. The optical system may be a catadioptric optical system having one or more lens elements. In one example, the optical system includes a single lens element and a retarder that is coated on a curved surface of the lens element. The retarder may be coated on an aspheric concave surface of the lens element. In another example the retarder may be coated on an aspheric convex surface of the lens element. One or more components of the optical system may be formed using a direct printing technique. This may allow for one or more adhesive layers and one or more hard coatings to be omitted from the optical system. A lens element may be directly printed on the display system to improve alignment between the optical system and the display system.

This application claims the benefit of provisional patent applicationNo. 62/993,505, filed Mar. 23, 2020, which is hereby incorporated byreference herein in its entirety.

BACKGROUND

This relates generally to optical systems and, more particularly, tooptical systems for head-mounted displays.

Head-mounted displays such as virtual reality glasses use lenses todisplay images for a user. A microdisplay may create images for each ofa user's eyes. A lens may be placed between each of the user's eyes anda portion of the microdisplay so that the user may view virtual realitycontent.

If care is not taken, a head-mounted display may be cumbersome andtiring to wear. Optical systems for head-mounted displays may usearrangements of lenses that are bulky and heavy. Extended use of ahead-mounted display with this type of optical system may beuncomfortable.

It would therefore be desirable to be able to provide improvedhead-mounted displays.

SUMMARY

A head-mounted display may include a display system and an opticalsystem. The display system and optical system may be supported by ahousing that is worn on a user's head. The head-mounted display may usethe display system and optical system to present images to the userwhile the housing is being worn on the user's head.

The display system may have a pixel array that produces image lightassociated with the images. The display system may also have a linearpolarizer through which image light from the pixel array passes and aquarter wave plate through which the light passes after passing throughthe linear polarizer.

The optical system may be a catadioptric optical system having a singlelens element. The single lens element may have a retarder that is coatedon a curved surface of the lens element. In one example the retarder maybe coated on an aspheric concave surface of the lens element. In anotherexample the retarder may be coated on an aspheric convex surface of thelens element. The retarder may be interposed between the lens elementand a partially reflective mirror.

In some cases, retarders may be formed on both sides of the lenselement. The cumulative retardation provided by the retarders mayeffectively form a quarter wave plate. In another possible arrangement,a single reflective polarizer and retarder layer may be included in theoptical system instead of a reflective polarizer and a separate retarderlayer. The reflective polarizer and retarder layer may optionallyprovide optical power.

In some cases, the optical system may be manufactured using one or moredirect 3D printing steps. In the 3D printing process, material for acomponent in the optical system (e.g., material for a lens element) maybe printed directly on the underlying layers in the stack. This type ofdirect printing process may be used for one or more components in theoptical system. This type of manufacturing technique may allow for oneor more adhesive layers and/or one or more hard coatings to be omittedfrom the optical system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of an illustrative head-mounted display inaccordance with an embodiment.

FIG. 2 is a diagram of an illustrative head-mounted display showingcomponents of an illustrative optical system in the head-mounted displayin accordance with an embodiment.

FIG. 3 is a cross-sectional side view of an illustrative head-mounteddisplay showing how the polarization of light changes when passingthrough the optical system of FIG. 2 in accordance with an embodiment.

FIG. 4 is a diagram of an illustrative head-mounted display showingcomponents of an illustrative optical system in the head-mounted displaywith a retarder on an aspheric convex surface in accordance with anembodiment.

FIG. 5 is a diagram of an illustrative head-mounted display showingcomponents of an illustrative optical system in the head-mounted displaywith a first retarder on an aspheric convex surface and a secondretarder on an aspheric concave surface in accordance with anembodiment.

FIG. 6 is a diagram of an illustrative head-mounted display showingcomponents of an illustrative optical system in the head-mounted displaywith a reflective polarizer and retarder layer in accordance with anembodiment.

FIG. 7 is a cross-sectional side view of an illustrative head-mounteddisplay showing how the polarization of light changes when passingthrough the optical system of FIG. 6 in accordance with an embodiment.

FIG. 8 is a diagram of an illustrative head-mounted display showingcomponents of an illustrative optical system in the head-mounted displaywith a planar reflective polarizer and retarder layer that providesoptical power in accordance with an embodiment.

FIG. 9 is a top view of an illustrative retarder layer with cutouts inaccordance with an embodiment.

FIG. 10 is a cross-sectional side view of an illustrative head-mounteddisplay showing components of an illustrative optical system with threelens elements, five adhesive layers, and five hard coatings, inaccordance with an embodiment.

FIG. 11 is a cross-sectional side view of an illustrative head-mounteddisplay showing components of an illustrative optical system with threelens elements, three adhesive layers, and two hard coatings, inaccordance with an embodiment.

FIG. 12 is a cross-sectional side view of an illustrative head-mounteddisplay showing components of an illustrative optical system with threelens elements, no adhesive layers, and two hard coatings, in accordancewith an embodiment.

FIG. 13 is a cross-sectional side view of an illustrative head-mounteddisplay showing components of an illustrative optical system with twolens elements, no adhesive layers, and two hard coatings, in accordancewith an embodiment.

FIG. 14 is a cross-sectional side view of an illustrative head-mounteddisplay showing components of an illustrative optical system with twolens elements, no adhesive layers, two hard coatings, and a linearpolarizer coated on one of the lens elements in accordance with anembodiment.

FIG. 15 is a cross-sectional side view of an illustrative head-mounteddisplay showing components of an illustrative optical system with onelens elements, no adhesive layers, and two hard coatings, in accordancewith an embodiment.

FIG. 16 is a cross-sectional side view of an illustrative head-mounteddisplay showing components of an illustrative optical system with a lenselement printed directly on the display panel in accordance with anembodiment.

FIG. 17 is a cross-sectional side view of an illustrative head-mounteddisplay showing components of an illustrative optical system with amicrolens array printed directly on the display panel and a lens elementprinted directly on the microlens array in accordance with anembodiment.

FIG. 18 is a cross-sectional side view of an illustrative head-mounteddisplay showing components of an illustrative optical system with threelens elements, three adhesive layers, two hard coatings, and two quarterwave plates in accordance with an embodiment.

DETAILED DESCRIPTION

Head-mounted displays may be used for virtual reality and augmentedreality systems. For example, a pair of virtual reality glasses that isworn on the head of a user may be used to provide a user with virtualreality content and/or augmented reality content.

An illustrative system in which an electronic device (e.g., ahead-mounted display such as a pair of virtual reality glasses) is usedin providing a user with virtual reality content is shown in FIG. 1 . Asshown in FIG. 1 , virtual reality glasses 10 (sometimes referred to asglasses 10, electronic device 10, head-mounted display 10, etc.) mayinclude a display system such as display system 40 that creates imagesand may have an optical system such as optical system 20 through which auser (see, e.g., user's eyes 46) may view the images produced by displaysystem 40 by looking in direction 48.

Display system 40 (sometimes referred to as display panel 40 or display40) may be based on a liquid crystal display, an organic light-emittingdiode display, an emissive display having an array of crystallinesemiconductor light-emitting diode dies, and/or displays based on otherdisplay technologies. Separate left and right displays may be includedin system 40 for the user's left and right eyes or a single display mayspan both eyes.

Visual content (e.g., image data for still and/or moving images) may beprovided to display system (display) 40 using control circuitry 42 thatis mounted in glasses (head-mounted display) 10 and/or control circuitrythat is mounted outside of glasses 10 (e.g., in an associated portableelectronic device, laptop computer, or other computing equipment).Control circuitry 42 may include storage such as hard-disk storage,volatile and non-volatile memory, electrically programmable storage forforming a solid-state drive, and other memory. Control circuitry 42 mayalso include one or more microprocessors, microcontrollers, digitalsignal processors, graphics processors, baseband processors,application-specific integrated circuits, and other processingcircuitry. Communications circuits in circuitry 42 may be used totransmit and receive data (e.g., wirelessly and/or over wired paths).Control circuitry 42 may use display system 40 to display visual contentsuch as virtual reality content (e.g., computer-generated contentassociated with a virtual world), pre-recorded video for a movie orother media, or other images. Illustrative configurations in whichcontrol circuitry 42 provides a user with virtual reality content usingdisplay system 40 may sometimes be described herein as an example. Ingeneral, however, any suitable content may be presented to a user bycontrol circuitry 42 using display system 40 and optical system 20 ofglasses 10.

Input-output devices 44 may be coupled to control circuitry 42.Input-output devices 44 may be used to gather user input from a user,may be used to make measurements on the environment surrounding glasses10, may be used to provide output to a user, and/or may be used tosupply output to external electronic equipment. Input-output devices 44may include buttons, joysticks, keypads, keyboard keys, touch sensors,track pads, displays, touch screen displays, microphones, speakers,light-emitting diodes for providing a user with visual output, sensors(e.g., a force sensors, temperature sensors, magnetic sensor,accelerometers, gyroscopes, and/or other sensors for measuringorientation, position, and/or movement of glasses 10, proximity sensors,capacitive touch sensors, strain gauges, gas sensors, pressure sensors,ambient light sensors, and/or other sensors). If desired, input-outputdevices 44 may include one or more cameras (e.g., cameras for capturingimages of the user's surroundings, cameras for performing gaze detectionoperations by viewing eyes 46, and/or other cameras).

FIG. 2 is a cross-sectional side view of glasses 10 showing how opticalsystem 20 and display system 40 may be supported by head-mounted supportstructures such as housing 12 for glasses 10. Housing 12 may have theshape of a frame for a pair of glasses (e.g., glasses 10 may resembleeyeglasses), may have the shape of a helmet (e.g., glasses 10 may form ahelmet-mounted display), may have the shape of a pair of goggles, or mayhave any other suitable housing shape that allows housing 12 to be wornon the head of a user. Configurations in which housing 12 supportsoptical system 20 and display system 40 in front of a user's eyes (e.g.,eyes 46) as the user is viewing system 20 and display system 40 indirection 48 may sometimes be described herein as an example. Ifdesired, housing 12 may have other desired configurations.

Housing 12 may be formed from plastic, metal, fiber-composite materialssuch as carbon-fiber materials, wood and other natural materials, glass,other materials, and/or combinations of two or more of these materials.

Input-output devices 44 and control circuitry 42 may be mounted inhousing 12 with optical system 20 and display system 40 and/or portionsof input-output devices 44 and control circuitry 42 may be coupled toglasses 10 using a cable, wireless connection, or other signal paths.

Display system 40 and the optical components of glasses 10 may beconfigured to display images for user 46 using a lightweight and compactarrangement. Optical system 20 may, for example, be based oncatadioptric lenses (e.g., lenses that use both reflecting andrefracting of light).

Display system 40 may include a source of images such as pixel array 14.Pixel array 14 may include a two-dimensional array of pixels P thatemits image light (e.g., organic light-emitting diode pixels,light-emitting diode pixels formed from semiconductor dies, liquidcrystal display pixels with a backlight, liquid-crystal-on-siliconpixels with a frontlight, etc.). A polarizer such as linear polarizer 16may be placed in front of pixel array 14 and/or may be laminated topixel array 14 to provide polarized image light. Linear polarizer 16 mayhave a pass axis aligned with the X-axis of FIG. 2 (as an example).Display system 40 may also include a wave plate such as quarter waveplate 18 to provide circularly polarized image light. The fast axis ofquarter wave plate 18 may be aligned at 45 degrees relative to the passaxis of linear polarizer 16. Quarter wave plate 18 may be mounted infront of polarizer 16 (between polarizer 16 and optical system 20). Ifdesired, quarter wave plate 18 may be attached to polarizer 16 (anddisplay 14).

Optical system 20 may include a lens element such as lens element 26.Lens element 26 may be formed from a transparent material such asplastic or glass. Lens element 26 may have a surface S1 that facesdisplay system 40 and a surface S2 that faces the user (e.g. eyes 46).Surface S1 may be a convex surface (e.g., a spherically convex surface,a cylindrically convex surface, or an aspherically convex surface) or aconcave surface (e.g., a spherically concave surface, a cylindricallyconcave surface, or an aspherically concave surface). Surface S2 may bea convex surface (e.g., a spherically convex surface, a cylindricallyconvex surface, or an aspherically convex surface) or a concave surface(e.g., a spherically concave surface, a cylindrically concave surface,or an aspherically concave surface). A spherically curved surface (e.g.,a spherically convex or spherically concave surface) may have a constantradius of curvature across the surface. In contrast, an asphericallycurved surface (e.g., an aspheric concave surface or an aspheric convexsurface) may have a varying radius of curvature across the surface. Acylindrical surface may only be curved about one axis instead of aboutmultiple axes as with the spherical surface. In one illustrativearrangement, shown in FIG. 2 , surface S1 is an aspheric convex surfaceand surface S2 is an aspheric concave surface. This arrangement may bedescribed as an example herein.

Optical structures such as partially reflective coatings, wave plates,reflective polarizers, linear polarizers, antireflection coatings,and/or other optical components may be incorporated into glasses 10(e.g., system 20, etc.). These optical structures may allow light raysfrom display system 40 to pass through and/or reflect from surfaces inoptical system 20 such as surfaces S1 and S2, thereby providing opticalsystem 20 with a desired lens power.

An illustrative arrangement for the optical layers is shown in FIG. 2 .First, the structural arrangement of these layers will be described. Thefunctionality of these layers will be discussed in more detail inconnection with FIG. 3 .

As shown in FIG. 2 , a partially reflective mirror (e.g., a metal mirrorcoating or other mirror coating such as a dielectric multilayer coatingwith a 50% transmission and a 50% reflection) such as partiallyreflective mirror 22 may be formed on the aspheric convex surface S1 oflens element 26. Partially reflective mirror 22 may sometimes bereferred to as beam splitter 22, half mirror 22, or partially reflectivelayer 22.

A wave plate such as wave plate 28 may be formed on the aspheric concavesurface S2 of lens element 26. Wave plate 28 (sometimes referred to asretarder 28, quarter wave plate 28, etc.) may be a quarter wave platethat conforms to surface S2 of lens element 26. Retarder 28 may be acoating on surface S2 of lens element 26.

Reflective polarizer 30 may be attached to retarder 28 using adhesivelayer 32. Reflective polarizer 30 may have orthogonal reflection andpass axes. Light that is polarized parallel to the reflection axis ofreflective polarizer 30 will be reflected by reflective polarizer 30.Light that is polarized perpendicular to the reflection axis andtherefore parallel to the pass axis of reflective polarizer 30 will passthrough reflective polarizer 30. Adhesive layer 32 may be a layer ofoptically clear adhesive (OCA).

Linear polarizer 34 may be attached to reflective polarizer 30 usingadhesive layer 36. Polarizer 34 may be referred to as an externalblocking linear polarizer 34. Linear polarizer 34 may have a pass axisaligned with the pass axis of reflective polarizer 30. Linear polarizer34 may have a pass axis that is orthogonal to the pass axis of linearpolarizer 16. Adhesive layer 36 may be a layer of optically clearadhesive (OCA).

One or more additional coatings 38 may also be included in opticalsystem 20 (sometimes referred to as lens 20, lens assembly 20, or lensmodule 20). Coatings 38 may include an anti-reflective coating (ARC),anti-smudge (AS) coating, or any other desired coatings.

FIG. 3 is a cross-sectional side view of an illustrative optical system20 and display system 40 showing how light from the display passesthrough the optical system of FIG. 2 . Note that the adhesive layers 32and 36 as well as coatings 38 are not shown in FIG. 3 since these layersdo not appreciably impact the polarization of light travelling throughthe system. As shown in FIG. 3 , a light ray R1 may be emitted fromdisplay 14. Light ray R1 exits display 14 having a mix of polarizationstates. As image light ray R1 exits display 14 and passes through linearpolarizer 16, ray R1 becomes linearly polarized in alignment with thepass axis of linear polarizer 16. The pass axis of linear polarizer 16may be, for example, aligned with the X-axis of FIG. 3 . After passingthrough polarizer 16, ray R2 passes through wave plate 18, which may bea quarter wave plate. As ray R2 passes through quarter wave plate 18,ray R3 exits the quarter wave plate circularly polarized (e.g., with aclockwise circular polarization).

When circularly polarized ray R3 strikes partially reflective mirror 22,a portion of ray R3 will pass through partially reflective mirror 22 tobecome reduced-intensity ray R4. Ray R4 will be refracted (partiallyfocused) by the shape of aspheric convex surface S1 of lens element 26.It should be noted that the depiction of surfaces of S1 and S2 as planarin FIG. 3 is merely illustrative. In practice, surfaces S1 and S2 may becurved (e.g., aspheric convex and aspheric concave) as discussed inconnection with FIG. 2 .

Wave plate 28 may convert the circular polarization of ray R4 intolinear polarization. Quarter wave plate 28 may, for example, convertcircularly polarized ray R4 into a ray R5 with a linear polarizationaligned with the X-axis of FIG. 2 . Quarter wave plate 28 in opticalsystem 20 may be rotated 90 degrees relative to quarter wave plate 18 indisplay 40 (e.g., the fast axes of quarter wave plates 18 and 28 areorthogonal).

As previously mentioned, reflective polarizer 30 may have orthogonalreflection and pass axes. Light that is polarized parallel to thereflection axis of reflective polarizer 30 will be reflected byreflective polarizer 30. Light that is polarized perpendicular to thereflection axis and therefore parallel to the pass axis of reflectivepolarizer 30 will pass through reflective polarizer 30. In theillustrative arrangement of FIG. 3 , reflective polarizer 30 has areflection axis that is aligned with the X-axis and a pass axis that isaligned with the Y-axis, so ray R5 will reflect from reflectivepolarizer 30 as reflected ray R6. It should be noted that the pass axisof reflective polarizer 30 is orthogonal to the pass axis of linearpolarizer 16 in display system 40.

Reflected ray R6 has a linear polarization aligned with the X-axis.After passing through quarter wave plate 28, the linear polarization ofray R6 will be converted into circular polarization (i.e., ray R6 willbecome counter-clockwise circularly polarized ray R7).

Circularly polarized ray R7 will travel through lens element 26 and aportion of ray R7 will be reflected in the positive Z direction by thepartially reflective mirror 22 on the convex surface S1 of lens element26 as reflected ray R8. The reflection from the curved shape of surfaceS1 provides optical system 20 with additional optical power. It shouldbe noted that any portion of ray R7 that is transmitted by the partiallyreflective layer 22 (e.g., R7′ in the negative Z-direction) may beconverted to a linear polarization by quarter wave plate 18 and thenreaches linear polarizer 16. This linearly polarized light has apolarization aligned with the Y-axis (e.g., orthogonal to the pass axisof linear polarizer 16) so that it is absorbed by linear polarizer 16.As a result, contrast degradation and stray light artifacts from thisportion of R7 are prevented in the image viewed by the user.

Ray R8 from partially reflective mirror 22 is converted from circularlypolarized light to linearly polarized light ray R9 by quarter wave plate28. Passing through the curved surface S2 of lens element 26 alsoprovides optical system 20 with additional optical power (e.g.,refractive optical power). The linear polarization of ray R9 is alignedwith the Y-axis, which is parallel to the pass axis of reflectivepolarizer 30. Accordingly, ray R9 will pass through reflective polarizer30 as ray R10 to provide a viewable image to the user.

Linear polarizer 34 has a pass axis aligned with the pass axis ofreflective polarizer 30 (i.e., parallel to the Y-axis in this example)so that any light from the external environment will be polarized bylinear polarizer 34 such that light is not reflected by the reflectivepolarizer 30. Any light that is transmitted by the linear polarizer 34and the reflective polarizer 30 will pass through retarders 28 and 18and be absorbed by linear polarizer 16. Linear polarizer 34 has a passaxis (parallel to the Y-axis) that is orthogonal to the pass axis(parallel to the X-axis) of linear polarizer 16 in the display.

The optical system 20 may be formed as a single, solid lens assemblywithout any intervening air gaps. Additionally, the lens assembly(without any intervening air gaps) includes only one lens element (26).As shown in FIG. 2 , each layer in optical system 20 is attacheddirectly to the adjacent layers. This is particularly noteworthy for thecase of retarder 28 being attached directly to the aspheric concavesurface S2 of lens element 26.

Conventionally, retarders are planar. However, in some embodiments,retarder 28 is a coating that is applied directly on the curved surfaceof lens element 26 to provide uniform retardation across the lenselement. Thereby, retarder 28 in FIG. 2 may have aspheric curvature(e.g., curvature along multiple axes and with different radii ofcurvature) with a relatively uniform thickness to provide a relativelyuniform retardation. Retardation is equal to the thickness of theretarder multiplied by the birefringence of the retarder material. Thethickness 62 (shown in FIG. 2 ) of retarder 28 may be relatively uniformacross the optical system (lens assembly). Retarder 28 conforms to thethree-dimensional surface of lens element 26 and may sometimes bereferred to as a coating (e.g., coating 28 or retarder coating 28).

As specific examples, the retardation provided by retarder 28 across theentire retarder may be uniform within 20%, within 10%, within 5%, within3%, within 2%, within 1%, etc. Similarly, the thickness 62 of retarder28 across the entire retarder may be uniform within 20%, within 10%,within 5%, within 3%, within 2%, within 1%, etc. In other words, theretardation variation across the retarder is no more than 20%, no morethan 10%, no more than 5%, no more than 3%, no more than 2%, no morethan 1%, etc. The thickness variation across the retarder is no morethan 20%, no more than 10%, no more than 5%, no more than 3%, no morethan 2%, no more than 1%, etc.

An alternate design to the highly uniform retarder 28 on the asphericsurface S2 of lens element 26 is having a planar surface S2 and anadditional lens element in the lens assembly. In this case, themulti-lens-element design is required for the desired optical power.However, the single lens element design of FIGS. 2 and 3 is a morecompact (thinner), lighter solution than a multiple lens element design.Having a compact, light lens design is valuable to improve the userexperience, particularly in head-mounted devices. The single lenselement design of FIGS. 2 and 3 may also have more curvature than amultiple lens element design. Having a single lens element also obviatesany issues of alignment between the multiple lens elements and mayresult in omission of other layers from the optical system (e.g., otheradhesive layers, retarders, etc.). As yet another benefit, the singlelens element module of FIGS. 2 and 3 has a lower materials/manufacturingcost due to the reduced number of layers and simplified manufacturingprocess.

Retarder 28 may be formed from any desired materials using any desiredprocesses. As one example, retarder 28 may be formed from a liquidcrystal material that is deposited over a photo-aligned alignment layer.As another example, retarder 28 may be formed from a liquid crystalmaterial that is aligned using shear alignment. As yet another example,retarder 28 may be formed from an inorganic material using obliquedeposition. The materials for retarder 28 may be deposited using spincoating, spray coating, physical vapor deposition (PVD), or any otherdesired techniques.

The example of a material having a uniform birefringence and relativelyuniform birefringence being used to form the retarder is merelyillustrative. Any type of retarder that provides uniform retardation maybe used. As one example, the retarder may have a first thickness and afirst birefringence in a first portion. The retarder may have a secondthickness and a second birefringence in a second portion. The secondbirefringence may be different than the first birefringence and thesecond thickness may be different than the first thickness. However, theretardation may be the same in both portions. In other words, theretarder may be provided with different birefringence in differentportions that are compensated by different thicknesses in the differentportions to provide uniform retardation. These types of techniques maybe used to provide uniform retardation even when uniform thickness isnot practical from a manufacturing standpoint.

FIG. 4 is a cross-sectional side view of an alternate design for opticalsystem 20. Optical system 20 in FIG. 4 is similar to the optical systemshown in connection with FIG. 2 . Optical system 20 in FIG. 4 includes alens element 26 having an aspheric convex surface S1 and an asphericconcave surface S2. On the side of lens element 26 facing eye 46 (e.g.,on surface S2), there is an adhesive layer 32, reflective polarizer 30,adhesive layer 36, linear polarizer 34, and coatings 38. On the side oflens element 26 facing display 14 (e.g., on surface S1), there is apartially reflective layer 22. These layers are all in the same relativepositions as in FIG. 2 .

However, in FIG. 2 , retarder 28 is positioned adjacent to S2 of lenselement 26. In particular, retarder 28 is interposed between adhesivelayer 32 and lens element 26. In contrast, in FIG. 4 , retarder 28 ispositioned adjacent to surface S1 of lens element 26. Retarder 28 inFIG. 4 is interposed between partially reflective layer 22 and lenselement 26.

The functionality of the optical layers in optical system 20 is the samein FIG. 4 as in FIG. 2 . However, in FIG. 4 the retarder 28 is on theconvex side of the lens element (instead of on the concave side of thelens element). Retarder 28 in FIG. 4 may be formed from the samematerials and may have the same retardation and thickness uniformitiesas discussed in connection with FIG. 2 . From a manufacturingstandpoint, forming a uniform thickness retarder may be easier on theconvex side of the lens element than on the concave side of the lenselement. Therefore, positioning retarder 28 on the convex surface S1 oflens element 26 may improve the ease and cost of manufacturing.

FIG. 5 is a cross-sectional side view of an alternate design for opticalsystem 20. Optical system 20 in FIG. 5 is similar to the optical systemshown in connection with FIG. 2 . Optical system 20 in FIG. 5 includes alens element 26 having an aspheric convex surface S1 and an asphericconcave surface S2. On the side of lens element 26 facing eye 46, thereis an adhesive layer 32, reflective polarizer 30, adhesive layer 36,linear polarizer 34, and coatings 38. On the side of lens element 26facing display 14, there is a partially reflective layer 22. Theselayers are all in the same relative positions as in FIG. 2 .

However, in FIG. 2 , optical system 20 includes only one retarder 28that is positioned adjacent to surface S2 of lens element 26. Incontrast, in FIG. 5 , optical system 20 includes first and secondretarders 28-1 and 28-2, with each retarder positioned on opposing sidesof the lens element. In particular, retarder 28-2 is positioned adjacentto surface S2 of lens element 26. Retarder 28-2 is interposed betweenadhesive layer 32 and lens element 26. Retarder 28-1 is positionedadjacent to surface S1 of lens element 26. Retarder 28-1 is interposedbetween partially reflective layer 22 and lens element 26.

The functionality of the optical layers in optical system 20 is the samein FIG. 5 as in FIG. 2 . However, in FIG. 5 the retarder 28 is splitbetween both sides of the lens element (instead of on one side of thelens element). In particular retarder 28-2 may be a compensatingretarder that compensates for retardation variations in retarder 28-1.Referring to one retarder as a compensating retarder is merely a matterof nomenclature. In general, each retarder may compensate for thevariations in the other retarder. The two retarders collectively providea uniform retardation across the optical system.

The two retarders of FIG. 5 may be able to account for manufacturingvariations in depositing each retarder individually. For example, whenforming retarder 28-1 on the convex side of the lens element, the edgesof the retarder may tend to be thicker than the center of the retarder.On the other hand, when forming retarder 28-2 on the concave side of thelens element, the center of the retarder may tend to be thicker than theedges of the retarder. Therefore, the thick center of retarder 28-2 willoverlap the thin center of retarder 28-1 and the thick edges of retarder28-1 will overlap the thin center of retarder 28-1. The cumulativethickness of retarders 28-1 and 28-2 may therefore be uniform across theoptical system.

As specific examples, the cumulative retardation provided by retarders28-1 and 28-2 across the entire optical system may be uniform within20%, within 10%, within 5%, within 3%, within 2%, within 1%, etc.Similarly, the cumulative thickness of retarders 28-1 and 28-2 acrossthe entire retarder may be uniform within 20%, within 10%, within 5%,within 3%, within 2%, within 1%, etc. In other words, the retardationvariation across the retarders is no more than 20%, no more than 10%, nomore than 5%, no more than 3%, no more than 2%, no more than 1%, etc.The thickness variation across the retarders is no more than 20%, nomore than 10%, no more than 5%, no more than 3%, no more than 2%, nomore than 1%, etc. Both retarders 28-1 and 28-2 in FIG. 5 may be formedfrom the same materials and processes as discussed in connection withFIG. 2 .

FIG. 6 is a cross-sectional side view of an alternate design for opticalsystem 20. Optical system 20 in FIG. 6 is similar to the optical systemshown in connection with FIG. 2 . Optical system 20 in FIG. 6 includes alens element 26 having an aspheric convex surface S1 and an asphericconcave surface S2. On the side of lens element 26 facing eye 46, thereis an adhesive layer 36, linear polarizer 34, and coatings 38. On theside of lens element 26 facing display 14, there is a partiallyreflective layer 22. These layers are all in the same relative positionsas in FIG. 2 .

However, in FIG. 2 , there is a retarder 28 on surface S2 of lenselement 26 and a separate reflective polarizer 30 that is coupled toretarder 28 using adhesive layer 32. In contrast, in FIG. 6 a singlereflective polarizer and retarder layer 72 is used instead of aseparately formed reflective polarizer and retarder. As shown in FIG. 6, reflective polarizer and retarder layer 72 (sometimes referred to ascircular reflective polarizer 72) is coated directly on surface S2 oflens element 26. Reflective polarizer and retarder layer 72 may reflectlight having a first circular polarization type and may transmit lighthaving a second, opposite circular polarization type. The lighttransmitted through reflective polarizer and retarder layer 72 may beconverted from circularly polarized light to linearly polarized light.

Reflective polarizer and retarder layer 72 may be formed fromcholesteric liquid crystal or any other desired materials. Theretardation provided by reflective polarizer and retarder layer 72 ontransmitted light may be uniform across the reflective polarizer andretarder layer 72. As specific examples, the retardation provided byreflective polarizer and retarder layer 72 across the entire reflectivepolarizer and retarder layer may be uniform within 20%, within 10%,within 5%, within 3%, within 2%, within 1%, etc. Similarly, thethickness of reflective polarizer and retarder layer 72 across theentire reflective polarizer and retarder layer may be uniform within20%, within 10%, within 5%, within 3%, within 2%, within 1%, etc. Inother words, the retardation variation across the reflective polarizerand retarder layer is no more than 20%, no more than 10%, no more than5%, no more than 3%, no more than 2%, no more than 1%, etc. Thethickness variation across the reflective polarizer and retarder layeris no more than 20%, no more than 10%, no more than 5%, no more than 3%,no more than 2%, no more than 1%, etc.

FIG. 7 is a cross-sectional side view of an illustrative optical system20 and display system 40 showing how light from the display passesthrough the optical system of FIG. 6 . Note that the adhesive layer 36as well as coatings 38 are not shown in FIG. 7 since these layers do notappreciably impact the polarization of light travelling through thesystem. As shown in FIG. 7 , a light ray R1 may be emitted from display14. Light ray R1 exits display 14 having a mix of polarization states.As image light ray R1 exits display 14 and passes through linearpolarizer 16, ray R1 becomes linearly polarized in alignment with thepass axis of linear polarizer 16. The pass axis of linear polarizer 16may be, for example, aligned with the X-axis of FIG. 7 . After passingthrough polarizer 16, ray R2 passes through wave plate 18, which may bea quarter wave plate. As ray R2 passes through quarter wave plate 18,ray R3 exits the quarter wave plate circularly polarized (e.g., with aclockwise circular polarization).

When circularly polarized ray R3 strikes partially reflective mirror 22,a portion of ray R3 will pass through partially reflective mirror 22 tobecome reduced-intensity ray R4. Ray R4 will be refracted (partiallyfocused) by the shape of aspheric convex surface S1 of lens element 26.It should be noted that the depiction of surfaces of S1 and S2 as planarin FIG. 7 is merely illustrative. In practice, surfaces S1 and S2 may becurved (e.g., aspheric) as discussed in connection with FIG. 2 .

Reflective polarizer and retardation layer 72 may reflect light havingclockwise circular polarization (a first circularly polarization) andmay transmit light having counter-clockwise circular polarization (asecond, opposite circular polarization). Accordingly, R4 is reflected byreflective polarizer and retardation layer 72. Reflected ray R5 passesthrough lens element 26 and a portion of ray R5 will be reflected in thepositive Z direction by the partially reflective mirror 22 on the convexsurface S1 of lens element 26 as reflected ray R6. The reflection fromthe curved shape of surface S1 provides optical system 20 withadditional optical power. It should be noted that any portion of ray R5that is transmitted by the partially reflective layer 22 (e.g., R5′ inthe negative Z-direction) may be converted to a linear polarization byquarter wave plate 18 and then reaches linear polarizer 16. Thislinearly polarized light has a polarization aligned with the Y-axis(e.g., orthogonal to the pass axis of linear polarizer 16) so that it isabsorbed by linear polarizer 16. As a result, contrast degradation andstray light artifacts from this portion of R5 are prevented in the imageviewed by the user.

Ray R6 from partially reflective mirror 22 is converted from circularlypolarized light to linearly polarized light ray R7 by reflectivepolarizer and retardation layer 72. Reflective polarizer and retardationlayer 72 transmits counter-clockwise circularly polarized light andconverts this light to linearly polarized light. Passing through thecurved surface S2 of lens element 26 also provides optical system 20with additional optical power.

The linear polarization of ray R7 is aligned with the Y-axis, which isparallel to the pass axis of linear polarizer 34. Linear polarizer 34has a pass axis aligned with the pass axis of reflective polarizer andretardation layer 72 (i.e., parallel to the Y-axis in this example) andwill therefore remove any residual non-Y-axis polarization from ray R7before ray R7 reaches viewers eye 46. Linear polarizer 34 has a passaxis (parallel to the Y-axis) that is orthogonal to the pass axis(parallel to the X-axis) of linear polarizer 16 in the display.

FIG. 8 is a cross-sectional side view of an alternate design for opticalsystem 20. Optical system 20 in FIG. 8 is similar to the optical systemshown in connection with FIG. 6 . Optical system 20 in FIG. 8 includes alens element 26 having an aspheric convex surface S1. On the side oflens element 26 facing eye 46, there is a reflective polarizer andretarder layer 72, adhesive layer 36, linear polarizer 34, coatings 38.On the side of lens element 26 facing display 14, there is a partiallyreflective layer 22. These layers are all in the same relative positionsas in FIG. 6 .

However, in FIG. 6 , surface S2 of lens element 26 is aspheric concave.In FIG. 8 , surface S2 of lens element 26 is planar. Reflectivepolarizer and retarder layer 72 is also planar but is patterned toprovide additional optical power. The function of the layers in theoptical system of FIG. 8 is the same as described in connection withFIGS. 6 and 7 (e.g., the polarization of light is manipulated in thesame way as in FIGS. 6 and 7 ). However, reflective polarizer andretarder layer 72 in FIG. 8 is patterned to have optical power.

Reflective polarizer and retarder layer 72 may be formed from apatterned material such as patterned cholesteric liquid crystal.Reflective polarizer and retarder layer 72 may be patterned to form aFresnel lens, as one example. Having reflective polarizer and retarderlayer 72 provide optical power allows for the same optical power to beachieved while having surface S2 be planar, which may improve the easeand cost of manufacturing. The retardation provided by reflectivepolarizer and retarder layer 72 on transmitted light may be uniformacross the reflective polarizer and retarder layer 72. As specificexamples, the retardation provided by reflective polarizer and retarderlayer 72 across the entire reflective polarizer and retarder layer maybe uniform within 20%, within 10%, within 5%, within 3%, within 2%,within 1%, etc. Similarly, the thickness of reflective polarizer andretarder layer 72 across the entire reflective polarizer and retarderlayer may be uniform within 20%, within 10%, within 5%, within 3%,within 2%, within 1%, etc. In other words, the retardation variationacross the reflective polarizer and retarder layer is no more than 20%,no more than 10%, no more than 5%, no more than 3%, no more than 2%, nomore than 1%, etc. The thickness variation across the reflectivepolarizer and retarder layer is no more than 20%, no more than 10%, nomore than 5%, no more than 3%, no more than 2%, no more than 1%, etc.

Any of the retarder layers in the aforementioned embodiments (e.g.,retarder 28 in FIGS. 2 and 4 , retarders 28-1 and 28-2 in FIG. 5 , andreflective polarizer and retarder layer 72 in FIGS. 6 and 8 ) may beformed using any desired techniques. For example, the retarder layer maybe formed as a coating (e.g., a liquid coating) on the lens element(e.g. using spin coating, spray coating, physical vapor deposition,etc.). Alternatively, a film-based retarder layer may be formed on thelens element (e.g., a solid retarder film may be laminated to the lenselement). In yet another example, the retarder may include a retarderlayer that is coated on a flat carrier (e.g., liquid material for theretarder layer is coated on a solid carrier). The flat carrier andretarder combination is then applied (e.g., laminated) to the curvedsurface of the lens element. In yet another example, a carrier layer maybe used for smoothing of the lens element (e.g., to cover any roughnessor imperfections in the lens element). For example, a carrier layer forthe retarder may be applied (e.g., coated or laminated) to the lenselement and then the retarder layer may be applied (e.g., coated orlaminated) to the carrier layer.

In cases where the retarder layer is formed as a film that is laminatedto the surface of lens element 26, the retarder layer may have cutoutsto improve the ability of the film to conform to the three-dimensionalsurface of the lens element. FIG. 9 is a top view of a retarder 28 withcutouts.

As shown in FIG. 9 , retarder 28 has a plurality of cutouts 28C. Eachcutout may be interposed between respective protruding portions 28P ofthe retarder. These cutouts allow the retarder to conform to a desiredthree-dimensional surface (e.g., an aspheric concave surface or asphericconvex surface). The number of cutouts and the shape of the cutouts inFIG. 9 is merely illustrative. In general, any desired number of cutoutsof any desired shape may be used. Additionally, an additional gapfilling material (e.g., having the same index of refraction as theretarder layer) may optionally be applied in the cutout areas to avoidthe cutouts being visible to the viewer.

It should be noted that the use of reference numeral ‘28’ in FIG. 9 ismerely illustrative. This technique may be used on any of theaforementioned retarder layers or other films (e.g., retarder 28 inFIGS. 2 and 4 , retarders 28-1 and 28-2 in FIG. 5 , and reflectivepolarizer and retarder layer 72 in FIGS. 6 and 8 ).

Optical system 20 may include only the single lens element 26 (e.g., andno additional lens elements). The optical system may sometimes bereferred to as a lens module or a lens stack. The optical systemincludes a plurality of optical layers coupled together without an airgap. In some cases, one or more additional lens elements (that areoptionally) separated from the lens module by air gaps may be includedin the head-mounted device for additional manipulation of light withinthe optical system.

The aforementioned examples of optical system 20 including only a singlelens element are merely illustrative. In some cases, the optical systemmay include more than one lens element (e.g., two lens elements, threelens elements, four elements, more than four elements, etc.).

FIG. 10 is a cross-sectional side view of an optical system 20 thatincludes three lens elements. As shown, optical system includes lenselement 26-1, lens element 26-2, and lens element 26-3. Each one of thelens elements may be formed from a transparent material such as plasticor glass. In the example of FIG. 10 , lens element 26-1 has a surface S1that faces display system 40 and a surface S2 that faces the user (e.g.,eye 46). Lens element 26-2 has a surface S3 that faces lens element 26-1and display system 40 and a surface S4 that faces the user (e.g., eye46). Lens element 26-5 has a surface S5 that faces lens elements26-1/26-2 and display system 40 and a surface S6 that faces the user(e.g., eye 46).

As shown in the example of FIG. 10 , surfaces S1, S3, and S5 (facing thedisplay system) may be convex surfaces whereas surfaces S2, S4, and S6(facing the user) may be concave surfaces. Each convex surface in theoptical system may be a spherically convex surface, a cylindricallyconvex surface, an aspherically convex surface, etc. Each concavesurface in the optical system may be a spherically concave surface, acylindrically concave surface, an aspherically concave surface, etc.

Optical structures such as partially reflective coatings, wave plates,reflective polarizers, linear polarizers, antireflection coatings,and/or other optical components may be incorporated into the opticalsystem of FIG. 10 . The same functional layers as described inconnection with FIG. 2 may be incorporated into the optical system ofFIG. 10 . Specifically, the optical system in FIG. 10 includes apartially reflective mirror 22, a wave plate 28, a reflective polarizer30, a linear polarizer 34, and coatings 38. Coatings 38 may include ananti-reflective coating and may sometimes be referred to asanti-reflective coating 38 or anti-reflective layer 38.

The order of the aforementioned functional layers relative to thedisplay system is the same in FIG. 10 as in FIG. 2 . In other words, inFIG. 2 , the optical system includes half mirror 22, wave plate 28,reflective polarizer 30, linear polarizer 34, and anti-reflectivecoating 38 in that order (moving from the display towards the viewer).Similarly, in FIG. 10 , the optical system includes half mirror 22, waveplate 28, reflective polarizer 30, linear polarizer 34, andanti-reflective coating 38 in that order (moving from the displaytowards the viewer). Therefore, even though the optical system of FIG.10 includes additional lens elements, hard coatings, and adhesive layersrelative to the optical system of FIG. 2 , the polarization state of thelight passing through the system will be the same in FIG. 10 as in FIG.2 (e.g., the light in FIG. 10 will have polarization states manipulatedsimilar to as shown in FIG. 3 , with additional refraction from theadditional lens elements).

In addition to the lens elements 26-1/26-2/26-3, half mirror 22, waveplate 28, reflective polarizer 30, linear polarizer 34, andanti-reflective coating 38, the optical system may include one or morehard coat layers 102 and one or more adhesive layers 104. The hard coatlayers 102 (sometimes referred to as hard coatings 102) may be used toprotect the lens elements from damage during assembly and operation ofthe electronic device. Each individual lens element may be producedseparately during manufacturing. The separate lens elements may then beassembled together with adhesive. In this type of manufacturing process,the hard coatings may protect the lens elements from damage during theassembly process. The adhesive layers 104 used to adhere the discretelens elements together may be optically clear adhesive (OCA) layers suchas liquid optically clear adhesive (LOCA) layers. The hard coatings 102and optically clear adhesive layers 104 may have a high transparency(greater than 80%, greater than 90%, greater than 95%, greater than 99%,greater than 99.9%, etc.) to avoid reducing the efficiency of thesystem.

In FIG. 10 , there are five hard coatings 102-1, 102-2, 102-3, 102-4,and 102-5. Hard coating 102-1 is adjacent to surface S1 of lens element26-1 (between lens element 26-1 and half mirror 22). Hard coating 102-2is adjacent to surface S2 of lens element 26-1 (between lens element26-1 and quarter wave plate 28). Hard coating 102-3 is adjacent toquarter wave plate 28 (also between quarter wave plate 28 and lenselement 26-1). Hard coating 102-4 is adjacent to surface S5 of lenselement 26-3 (between lens element 26-3 and linear polarizer 34). Hardcoating 102-5 is positioned adjacent to surface S6 of lens element 26-3(between lens element 26-3 and anti-reflective coating 38).

In FIG. 10 , there are five adhesive layers. Adhesive layer 104-1 ispositioned between hard coatings 102-2 and 102-3. Adhesive layer 104-2is positioned adjacent to surface S3 of lens element 26-2 (betweenquarter wave plate 28 and lens element 26-2). Adhesive layer 104-3 ispositioned adjacent to surface S4 of lens element 26-2 (between lenselement 26-2 and reflective polarizer 30). Adhesive layer 104-4 ispositioned between reflective polarizer 30 and linear polarizer 34.Adhesive layer 104-5 is positioned between linear polarizer 34 and hardcoating 102-4.

The example of an optical system shown in FIG. 10 is merelyillustrative. In some cases, the optical system may be manufacturedusing one or more direct 3D printing or 3D forming steps. In the 3Dprinting process, material for a component in the optical system (e.g.,material for a lens element) may be printed directly on the underlyinglayers in the stack. This type of direct printing process may be usedfor one or more components in the optical system.

There are many benefits to using direct 3D printing during manufacturingof the optical system. One or more adhesive layers may be omitted fromthe optical system (since the layers are formed together directly and donot need to be attached with a separate adhesive). Omitting adhesivelayers in the optical system reduces the material cost of the opticalsystem. Additionally, achieving bubble-free adhesive layers in theoptical system may be challenging. Therefore, omitting adhesive layersalso simplifies the manufacturing cost and complexity of the opticalsystem.

Using direct 3D printing during manufacturing of the optical systemreduces the amount of handling of individual lens elements. This allowsfor one or more hard coatings in the stack to be omitted. Omitting hardcoatings in the optical system reduces the cost and manufacturingcomplexity of the optical system.

Omitting adhesive layers and/or hard coatings reduces the weight andthickness of the optical system. Weight is an important performancemetric for head-mounted devices to ensure comfortable operation by auser. Space in a head-mounted device may also be at a premium, soreducing the thickness of the optical system is helpful.

Using direct printing of one or more components in the optical systemalso improves the lens assembly and alignment process for the opticalsystem. A single lens assembly station may be used with each componentformed on the underlying component in a bottom-up manner. This type oftechnique allows each component in the optical system to be more easilyaligned with its adjacent components.

FIGS. 11-17 are cross-sectional side views of illustrative opticalsystems that are formed using one or more direct printing steps,allowing for one or more adhesive layers or hard coatings to be omittedrelative to the example of FIG. 10 .

In the example of FIG. 11 , two of the adhesive layers and three of thehard coatings are omitted relative to FIG. 10 (e.g., the optical systemin FIG. 11 includes 3 adhesive layers and 2 hard coatings). Because thecomponents are assembled in a bottom-up fashion in FIG. 11 , only twohard coatings are used (e.g., hard coating 102-1 adjacent to surface S1of lens element 26-1 and hard coating 102-2 adjacent to surface S6 oflens element 26-3). In the example of FIG. 11 , adhesive layer 104-1 maybe formed directly on surface S2 of lens element 26-1 (e.g., in directcontact without an air gap) and quarter wave plate 28 may be formeddirectly on the adhesive layer 104-1. In other words, hard coatings102-2 and 102-3 from FIG. 10 may be omitted in FIG. 11 , with no hardcoatings between lens elements 26-1 and 26-2 in FIG. 11 . Additionally,hard coating 102-4 from FIG. 10 may be omitted in FIG. 11 . Three totalhard coatings are therefore omitted in FIG. 11 relative to FIG. 10 .

Lens element 26-2 may be directly printed on quarter wave plate 28(e.g., using a 3D printing or 3D forming process). Thus, in the finalstack, there is no adhesive layer between lens element 26-2 and quarterwave plate 28. Lens element 26-2 (e.g., surface S3) and quarter waveplate 28 may be in direct contact without an intervening air gap.

Lens element 26-3 may be directly printed on linear polarizer 34 (e.g.,using a 3D printing or 3D forming process). Thus, in the final stack,there is no adhesive layer between lens element 26-3 and linearpolarizer 34. Lens element 26-3 (e.g., surface S5) and linear polarizer34 may be in direct contact without an intervening air gap. Two totaladhesive layers are therefore omitted in FIG. 11 relative to FIG. 10 .

In the example of FIG. 12 , all five of the adhesive layers and three ofthe hard coatings are omitted relative to FIG. 10 (e.g., the opticalsystem in FIG. 12 includes 0 adhesive layers and 2 hard coatings).Because the components are assembled in a bottom-up fashion in FIG. 12 ,only two hard coatings are used (e.g., hard coating 102-1 adjacent tosurface S1 of lens element 26-1 and hard coating 102-2 adjacent tosurface S6 of lens element 26-3).

In FIG. 11 , three adhesive layers are included in the optical system.Adhesive layer 104-1 attaches lens element 26-1 to quarter wave plate28. Adhesive layer 104-2 attaches reflective polarizer 30 to lenselement 26-2. Adhesive layer 104-3 attaches linear polarizer 34 toreflective polarizer 30.

In FIG. 12 , quarter wave plate 28, reflective polarizer 30, and linearpolarizer 34 are formed using a direct 3D printing process, obviatingthe need for adhesive layers. As shown in FIG. 12 , quarter wave plate28 is formed directly on surface S2 of lens element 26-1 without anintervening adhesive layer. Reflective polarizer 30 is formed directlyon surface S4 of lens element 26-2 without an intervening adhesivelayer. Linear polarizer 34 is formed directly on reflective polarizer 30without an intervening adhesive layer. There are therefore no adhesivelayers included in the optical system.

The example in FIGS. 11 and 12 of the optical system having three lenselements is merely illustrative. In another example, shown in FIG. 13 ,the first and second lens elements of FIG. 12 may be merged into asingle integrated lens element 26-1. A second lens element 26-2 is alsoincluded.

Quarter wave plate 28 may be formed as a coating on surface S2 of lenselement 26-1. The quarter wave plate 28 may be formed using a direct 3Dprinting or coating technique, obviating the need for adhesive betweenthe lens element and the quarter wave plate. The rest of the opticalsystem in FIG. 13 has a similar arrangement as in FIG. 12 , withreflective polarizer 30 and linear polarizer formed between quarter waveplate 28 and lens element 26-2 without any intervening adhesive layers.The quarter wave plate 28 may be a coating that is applied directly onthe curved surface S2 of lens element 26-1 to provide uniformretardation across the lens element (e.g., using the quarter wave platecoating techniques previously discussed). Quarter wave plate 28 may havea relatively uniform thickness across the surface S2.

As specific examples, the retardation provided by retarder 28 across theentire retarder may be uniform within 20%, within 10%, within 5%, within3%, within 2%, within 1%, etc. Similarly, the thickness of retarder 28across the entire retarder may be uniform within 20%, within 10%, within5%, within 3%, within 2%, within 1%, etc. In other words, theretardation variation across the retarder is no more than 20%, no morethan 10%, no more than 5%, no more than 3%, no more than 2%, no morethan 1%, etc. The thickness variation across the retarder is no morethan 20%, no more than 10%, no more than 5%, no more than 3%, no morethan 2%, no more than 1%, etc.

In another example, shown in FIG. 14 , the linear polarizer 34 may beintegrated with lens element 26-2. The linear polarizer may be directlyprinted on or integrated into lens 26-2 (e.g., using a direct 3Dprinting technique). In FIG. 14 , linear polarizer 34 is depicted asbeing printed on surface S4 of lens element 26-2. This example is merelyillustrative. Linear polarizer 34 may alternatively be printed onsurface S3 of lens element 26-2.

In another example, shown in FIG. 15 , the first and second lenselements of FIGS. 13 and 14 may be merged into a single integrated lenselement 26. As shown, quarter wave plate 28 may be formed as a coatingon surface S2 of lens element 26. The quarter wave plate 28 may beformed using a direct 3D printing technique, obviating the need foradhesive between the lens element and the quarter wave plate. Reflectivepolarizer 30 and linear polarizer 34 are formed between quarter waveplate 28 and hard coating 102-2 without any intervening adhesive layers.The quarter wave plate 28 may be a coating that is applied directly onthe curved surface of lens element 26 to provide uniform retardationacross the lens element (as previously discussed).

The optical systems of FIGS. 11-15 therefore all have a reduced numberof components relative to the optical system of FIG. 10 . This resultsin reduced manufacturing cost and complexity for the optical system.

In another possible embodiment for optical system 20, shown in FIG. 16 ,a lens element 26-1 may be printed directly on the display panel.Subsequent components may be attached to the lens element in a bottom-upmanner, similar to as discussed in connection with FIGS. 11-15 . Havinga lens element printed directly on the display panel as in FIG. 16 mayimprove alignment between the optical system 20 and the display panel40. Without the lens element 26-1 printed directly on display panel 40,the optical system 20 has to be aligned with display panel 40 after theoptical system is assembled. By forming the optical system directly onpanel 40 during manufacturing, the alignment between the optical systemand display panel 40 is greatly simplified.

As shown in FIG. 16 , lens element 26-1 may be applied to directly todisplay panel 40 (e.g., using a direct 3D printing technique). In otherwords, surface S1 of lens element 26-1 may be in direct contact withdisplay panel 40 without an intervening air gap.

The rest of the optical system has a similar arrangement as shown inFIG. 11 , with lens element 26-2 in FIG. 16 having the same propertiesas lens element 26-1 in FIG. 11 , lens element 26-3 in FIG. 16 havingthe same properties as lens element 26-2 in FIG. 11 , and lens element26-4 in FIG. 16 having the same properties as lens element 26-3 in FIG.11 . Using the lens element 26-1 printed on the display (as in FIG. 16 )allows for hard coating 102-1 in FIG. 11 to be omitted from the opticalsystem of FIG. 16 . In other words, there is no hard coating betweenlens element 26-2 and display system 40 in the optical system of FIG. 16. In the optical system of FIG. 16 , only one hard coating 102 isincluded (adjacent to surface S8 of lens element 26-4).

Lens element 26-1 may have a concave surface S2 that conforms to theconvex surface of lens element 26-2 and a surface S1 (e.g., a planarsurface) that conforms to the upper surface of display panel 40. Surface51 may have curvature (e.g., concave or convex curvature) in the eventthat display panel 40 is curved.

To summarize, including an additional lens element directly adjacent tothe display panel 40 improves the alignment of the optical system andthe display system. Additionally, the additional lens element directlyadjacent to the display allows for a hard coating within the opticalsystem to be omitted.

Lens elements 26-1 and 26-2 may be formed from the same material and mayhave the same index of refraction. In this case, the light from displaypanel 40 will not be refracted at the interface between lens elements26-1 and 26-2. Alternatively, lens elements 26-1 and 26-2 may be formedfrom different materials having different indices of refraction. In thistype of arrangement, light from the display panel 40 will be refractedat the interface between lens elements 26-1 and 26-2. In other words,the lens element 26-1 will provide additional lens power at theinterface between lens elements 26-1 and 26-2.

In another embodiment, shown in FIG. 17 , it may be desirable to includea microlens array 106 over display panel 40. The microlens array mayinclude a plurality of microlenses, with each microlens having a curvedupper surface and configured to focus light from the display. Includingmicrolens array 106 in the display system may increase the efficiency ofthe display system. Microlens array 106 may be referred to as being partof display panel 40. Alternatively, microlens array 106 may be referredto as being formed over display panel 40. It should be understood thatdisplay panel 40 in FIGS. 10-17 may include a pixel array 14, linearpolarizer 16, and wave plate 18 (e.g., as shown in FIG. 2 ). Themicrolens array 106 may, as an example, be formed over the quarter waveplate 18 (e.g., between quarter wave plate 18 and lens element 26-1).

During manufacturing, microlens array 106 may formed on the displaysystem 40 using a 3D printing process. Subsequently, lens element 26-1may be formed over the microlens array, also using a 3D printingprocess. This example is merely illustrative. In another possibleexample, the display system 40 may be separately formed to includemicrolens array 106. In this case, the lens element 26-1 deposition maybe the first 3D printing step of the bottom-up optical systemmanufacturing.

FIGS. 16 and 17 show examples of having a lens element of optical system20 printed directly on the display system 40. The rest of optical system20 in FIGS. 16 and 17 is the same as the optical system in FIG. 11 . Itshould be noted that this example is merely illustrative. In general,the panel printing technique (where a lens element is printed on thedisplay panel for improved alignment) of FIGS. 16 and 17 may be usedwith an optical system that has the arrangement of FIG. 11 , FIG. 12 ,FIG. 13 , FIG. 14 , FIG. 15 , or any other desired optical systemarrangement. In each of FIGS. 11-15 , an additional lens element (andoptionally a microlens array) may be printed on the display panelbetween the display panel and lens element 26-1 to implement the panelprinting technique of FIG. 16 or 17 .

An additional quarter wave plate may optionally be included in theoptical system. FIG. 18 is a cross-sectional side view of anillustrative optical system having two quarter wave plates. The opticalsystem of FIG. 18 has the same arrangement as the optical system in FIG.11 , plus two additional layers. In addition to first quarter wave plate28 (formed between lens elements 26-2 and lens elements 26-1), a secondquarter wave plate 108 is formed between linear polarizer 34 and lenselement 26-3. An additional anti-reflective coating may also be includedin the optical system. Anti-reflective coating 110 may be formed onsurface S5 of lens element 26-3 when quarter wave plate 108 is present.Lens element 26-3 is therefore interposed between anti-reflectivecoatings 110 and 38.

Quarter wave plate 108 may mitigate reflections from interfaces above S5of lens element 26-3 (e.g., from S6 of lens element 26-3, the user'seyes, etc.). Mitigating reflections in this way beneficially increasesthe contrast ratio in the system.

Quarter wave plate 108 may be a QWP film that is 3D formed betweenlinear polarizer 34 and lens element 26-3. Alternatively, quarter waveplate 108 may be formed from a liquid material that is coated on theinterface between linear polarizer 34 and lens element 26-3. As anotheroption, quarter wave plate 108 may be formed from a liquid material thatis 3D printed on the interface between linear polarizer and lens element26-3. In general, quarter wave plate 108 may be formed from any desiredmaterial and may be applied using any desired techniques.

FIG. 18 shows the additional quarter wave plate being applied to thesystem of FIG. 11 . However, it should be understood that the additionalquarter wave plate (interposed between the user and linear polarizer 34)and optional additional anti-reflective coating (interposed between theadditional quarter wave plate and a lens element) may be applied to anysystem (e.g., any of the systems shown in FIGS. 10-17 ).

In FIGS. 10-18 , spaces are shown between some of the adjacent layers inoptical system 20. It should be understood that this is merely forillustration purposes. In practice, each layer in optical system 20 maybe in direct contact with its adjacent layers such that no air gaps arepresent in the optical system and the optical system 20 is formed as asingle, solid lens assembly. One or more air gaps may optionally beformed between layers in the optical system if desired.

A physical environment refers to a physical world that people can senseand/or interact with without aid of electronic systems. Physicalenvironments, such as a physical park, include physical articles, suchas physical trees, physical buildings, and physical people. People candirectly sense and/or interact with the physical environment, such asthrough sight, touch, hearing, taste, and smell.

In contrast, a computer-generated reality (CGR) environment refers to awholly or partially simulated environment that people sense and/orinteract with via an electronic system (e.g., an electronic systemincluding the display systems described herein). In CGR, a subset of aperson's physical motions, or representations thereof, are tracked, and,in response, one or more characteristics of one or more virtual objectssimulated in the CGR environment are adjusted in a manner that comportswith at least one law of physics. For example, a CGR system may detect aperson's head turning and, in response, adjust graphical content and anacoustic field presented to the person in a manner similar to how suchviews and sounds would change in a physical environment. In somesituations (e.g., for accessibility reasons), adjustments tocharacteristic(s) of virtual object(s) in a CGR environment may be madein response to representations of physical motions (e.g., vocalcommands).

A person may sense and/or interact with a CGR object using any one oftheir senses, including sight, sound, touch, taste, and smell. Forexample, a person may sense and/or interact with audio objects thatcreate 3D or spatial audio environment that provides the perception ofpoint audio sources in 3D space. In another example, audio objects mayenable audio transparency, which selectively incorporates ambient soundsfrom the physical environment with or without computer-generated audio.In some CGR environments, a person may sense and/or interact only withaudio objects. Examples of CGR include virtual reality and mixedreality.

A virtual reality (VR) environment refers to a simulated environmentthat is designed to be based entirely on computer-generated sensoryinputs for one or more senses. A VR environment comprises a plurality ofvirtual objects with which a person may sense and/or interact. Forexample, computer-generated imagery of trees, buildings, and avatarsrepresenting people are examples of virtual objects. A person may senseand/or interact with virtual objects in the VR environment through asimulation of the person's presence within the computer-generatedenvironment, and/or through a simulation of a subset of the person'sphysical movements within the computer-generated environment.

In contrast to a VR environment, which is designed to be based entirelyon computer-generated sensory inputs, a mixed reality (MR) environmentrefers to a simulated environment that is designed to incorporatesensory inputs from the physical environment, or a representationthereof, in addition to including computer-generated sensory inputs(e.g., virtual objects). On a virtuality continuum, a mixed realityenvironment is anywhere between, but not including, a wholly physicalenvironment at one end and virtual reality environment at the other end.

In some MR environments, computer-generated sensory inputs may respondto changes in sensory inputs from the physical environment. Also, someelectronic systems for presenting an MR environment may track locationand/or orientation with respect to the physical environment to enablevirtual objects to interact with real objects (that is, physicalarticles from the physical environment or representations thereof). Forexample, a system may account for movements so that a virtual treeappears stationery with respect to the physical ground. Examples ofmixed realities include augmented reality and augmented virtuality.

An augmented reality (AR) environment refers to a simulated environmentin which one or more virtual objects are superimposed over a physicalenvironment, or a representation thereof. For example, an electronicsystem for presenting an AR environment may have a transparent ortranslucent display through which a person may directly view thephysical environment. The system may be configured to present virtualobjects on the transparent or translucent display, so that a person,using the system, perceives the virtual objects superimposed over thephysical environment. Alternatively, a system may have an opaque displayand one or more imaging sensors that capture images or video of thephysical environment, which are representations of the physicalenvironment. The system composites the images or video with virtualobjects, and presents the composition on the opaque display. A person,using the system, indirectly views the physical environment by way ofthe images or video of the physical environment, and perceives thevirtual objects superimposed over the physical environment. As usedherein, a video of the physical environment shown on an opaque displayis called “pass-through video,” meaning a system uses one or more imagesensor(s) to capture images of the physical environment, and uses thoseimages in presenting the AR environment on the opaque display. Furtheralternatively, a system may have a projection system that projectsvirtual objects into the physical environment, for example, as ahologram or on a physical surface, so that a person, using the system,perceives the virtual objects superimposed over the physicalenvironment.

An augmented reality environment also refers to a simulated environmentin which a representation of a physical environment is transformed bycomputer-generated sensory information. For example, in providingpass-through video, a system may transform one or more sensor images toimpose a select perspective (e.g., viewpoint) different than theperspective captured by the imaging sensors. As another example, arepresentation of a physical environment may be transformed bygraphically modifying (e.g., enlarging) portions thereof, such that themodified portion may be representative but not photorealistic versionsof the originally captured images. As a further example, arepresentation of a physical environment may be transformed bygraphically eliminating or obfuscating portions thereof.

An augmented virtuality (AV) environment refers to a simulatedenvironment in which a virtual or computer generated environmentincorporates one or more sensory inputs from the physical environment.The sensory inputs may be representations of one or more characteristicsof the physical environment. For example, an AV park may have virtualtrees and virtual buildings, but people with faces photorealisticallyreproduced from images taken of physical people. As another example, avirtual object may adopt a shape or color of a physical article imagedby one or more imaging sensors. As a further example, a virtual objectmay adopt shadows consistent with the position of the sun in thephysical environment.

There are many different types of electronic systems that enable aperson to sense and/or interact with various CGR environments. Examplesinclude head mounted systems, projection-based systems, heads-updisplays (HUDs), vehicle windshields having integrated displaycapability, windows having integrated display capability, displaysformed as lenses designed to be placed on a person's eyes (e.g., similarto contact lenses), headphones/earphones, speaker arrays, input systems(e.g., wearable or handheld controllers with or without hapticfeedback), smartphones, tablets, and desktop/laptop computers. A headmounted system may have one or more speaker(s) and an integrated opaquedisplay. Alternatively, a head mounted system may be configured toaccept an external opaque display (e.g., a smartphone). The head mountedsystem may incorporate one or more imaging sensors to capture images orvideo of the physical environment, and/or one or more microphones tocapture audio of the physical environment. Rather than an opaquedisplay, a head mounted system may have a transparent or translucentdisplay. The transparent or translucent display may have a mediumthrough which light representative of images is directed to a person'seyes. The display may utilize digital light projection, OLEDs, LEDs,uLEDs, liquid crystal on silicon, laser scanning light source, or anycombination of these technologies. The medium may be an opticalwaveguide, a hologram medium, an optical combiner, an optical reflector,or any combination thereof. In one embodiment, the transparent ortranslucent display may be configured to become opaque selectively.Projection-based systems may employ retinal projection technology thatprojects graphical images onto a person's retina. Projection systemsalso may be configured to project virtual objects into the physicalenvironment, for example, as a hologram or on a physical surface. Thedisplay systems described herein may be used for these types of systemsand for any other desired display arrangements.

As described above, one aspect of the present technology is thegathering and use of information such as information from input-outputdevices. The present disclosure contemplates that in some instances,data may be gathered that includes personal information data thatuniquely identifies or can be used to contact or locate a specificperson. Such personal information data can include demographic data,location-based data, telephone numbers, email addresses, twitter ID's,home addresses, data or records relating to a user's health or level offitness (e.g., vital signs measurements, medication information,exercise information), date of birth, username, password, biometricinformation, or any other identifying or personal information.

The present disclosure recognizes that the use of such personalinformation, in the present technology, can be used to the benefit ofusers. For example, the personal information data can be used to delivertargeted content that is of greater interest to the user. Accordingly,use of such personal information data enables users to calculatedcontrol of the delivered content. Further, other uses for personalinformation data that benefit the user are also contemplated by thepresent disclosure. For instance, health and fitness data may be used toprovide insights into a user's general wellness, or may be used aspositive feedback to individuals using technology to pursue wellnessgoals.

The present disclosure contemplates that the entities responsible forthe collection, analysis, disclosure, transfer, storage, or other use ofsuch personal information data will comply with well-established privacypolicies and/or privacy practices. In particular, such entities shouldimplement and consistently use privacy policies and practices that aregenerally recognized as meeting or exceeding industry or governmentalrequirements for maintaining personal information data private andsecure. Such policies should be easily accessible by users, and shouldbe updated as the collection and/or use of data changes. Personalinformation from users should be collected for legitimate and reasonableuses of the entity and not shared or sold outside of those legitimateuses. Further, such collection/sharing should occur after receiving theinformed consent of the users. Additionally, such entities shouldconsider taking any needed steps for safeguarding and securing access tosuch personal information data and ensuring that others with access tothe personal information data adhere to their privacy policies andprocedures. Further, such entities can subject themselves to evaluationby third parties to certify their adherence to widely accepted privacypolicies and practices. In addition, policies and practices should beadapted for the particular types of personal information data beingcollected and/or accessed and adapted to applicable laws and standards,including jurisdiction-specific considerations. For instance, in theUnited States, collection of or access to certain health data may begoverned by federal and/or state laws, such as the Health InsurancePortability and Accountability Act (HIPAA), whereas health data in othercountries may be subject to other regulations and policies and should behandled accordingly. Hence different privacy practices should bemaintained for different personal data types in each country.

Despite the foregoing, the present disclosure also contemplatesembodiments in which users selectively block the use of, or access to,personal information data. That is, the present disclosure contemplatesthat hardware and/or software elements can be provided to prevent orblock access to such personal information data. For example, the presenttechnology can be configured to allow users to select to “opt in” or“opt out” of participation in the collection of personal informationdata during registration for services or anytime thereafter. In anotherexample, users can select not to provide certain types of user data. Inyet another example, users can select to limit the length of timeuser-specific data is maintained. In addition to providing “opt in” and“opt out” options, the present disclosure contemplates providingnotifications relating to the access or use of personal information. Forinstance, a user may be notified upon downloading an application (“app”)that their personal information data will be accessed and then remindedagain just before personal information data is accessed by the app.

Moreover, it is the intent of the present disclosure that personalinformation data should be managed and handled in a way to minimizerisks of unintentional or unauthorized access or use. Risk can beminimized by limiting the collection of data and deleting data once itis no longer needed. In addition, and when applicable, including incertain health related applications, data de-identification can be usedto protect a user's privacy. De-identification may be facilitated, whenappropriate, by removing specific identifiers (e.g., date of birth,etc.), controlling the amount or specificity of data stored (e.g.,collecting location data at a city level rather than at an addresslevel), controlling how data is stored (e.g., aggregating data acrossusers), and/or other methods.

Therefore, although the present disclosure broadly covers use ofinformation that may include personal information data to implement oneor more various disclosed embodiments, the present disclosure alsocontemplates that the various embodiments can also be implementedwithout the need for accessing personal information data. That is, thevarious embodiments of the present technology are not renderedinoperable due to the lack of all or a portion of such personalinformation data.

The foregoing is merely illustrative and various modifications can bemade to the described embodiments. The foregoing embodiments may beimplemented individually or in any combination.

What is claimed is:
 1. A head-mounted display configured to displayimages viewable by a user, comprising: a display panel configured toproduce light for the images; and a lens module that receives the lightfrom the display panel, wherein the lens module comprises: first,second, and third lens elements, wherein each one of the first, second,and third lens elements has a convex surface and a concave surface; apartially reflective mirror that is interposed between the first lenselement and the display panel; a quarter wave plate that is formedbetween the first and second lens elements, wherein there is no adhesivelayer between the quarter wave plate and the second lens element; areflective polarizer that is formed between the second and third lenselements; and a linear polarizer that is formed between the second andthird lens elements, wherein there is no adhesive layer between thelinear polarizer and the third lens element.
 2. The head-mounted displaydefined in claim 1, wherein the first lens element is interposed betweenthe second lens element and the display panel and wherein the secondlens element is interposed between the first lens element and the thirdlens element.
 3. The head-mounted display defined in claim 2, whereinthe linear polarizer is interposed between the third lens element andthe reflective polarizer.
 4. The head-mounted display defined in claim1, wherein there is no adhesive layer between the reflective polarizerand the linear polarizer.
 5. The head-mounted display defined in claim1, wherein there is no adhesive layer between the reflective polarizerand the second lens element.
 6. The head-mounted display defined inclaim 1, wherein the lens module further comprises: a first hard coatingthat is interposed between the first lens element and the display panel;and a second hard coating, wherein the third lens element is interposedbetween the second hard coating and the linear polarizer and wherein thefirst and second hard coatings are the only hard coatings included inthe lens module.
 7. The head-mounted display defined in claim 1, furthercomprising: an additional quarter wave plate, wherein the linearpolarizer is interposed between the quarter wave plate and theadditional quarter wave plate.
 8. A head-mounted display configured todisplay images viewable by a user, comprising: a display panelconfigured to produce light for the images, wherein the display panelhas an upper surface; and a lens module that receives the light from thedisplay panel, wherein the lens module comprises: a first lens elementthat is formed directly on the upper surface of the display panel,wherein the first lens element has first and second opposing surfacesand wherein the first surface conforms to the upper surface of thedisplay panel; a second lens element that has third and fourth opposingsurfaces, wherein the second surface of the first lens element conformsto the third surface of the second lens element; a partially reflectivemirror that is interposed between the first and second lens elements; aquarter wave plate, wherein the second lens element is interposedbetween the quarter wave plate and the first lens element; a reflectivepolarizer, wherein the quarter wave plate is interposed between thereflective polarizer and the second lens element; and a linearpolarizer, wherein the reflective polarizer is interposed between thelinear polarizer and the quarter wave plate.
 9. The head-mounted displaydefined in claim 8, wherein the display panel includes a microlens arrayand wherein the first lens element is interposed between the microlensarray and the partially reflective mirror.
 10. The head-mounted displaydefined in claim 8, wherein the lens module further comprises: a thirdlens element that is interposed between the quarter wave plate and thereflective polarizer; and a fourth lens element, wherein the linearpolarizer is interposed between the fourth lens element and thereflective polarizer.
 11. A head-mounted display configured to displayimages viewable by a user, comprising: an array of pixels configured toproduce the images; a linear polarizer through which light associatedwith the images passes; a quarter wave plate that receives the lightfrom the linear polarizer; a lens module that receives the light fromthe quarter wave plate, wherein the lens module comprises: a lenselement having first and second opposing surfaces, wherein the firstsurface is an aspheric convex surface and wherein the second surface isan aspheric concave surface; a partially reflective mirror that isinterposed between the lens element and the quarter wave plate; and aretarder coating that is formed on a selected one of the first andsecond surfaces of the lens element.
 12. The head-mounted displaydefined in claim 11, wherein the lens module further comprises: areflective polarizer, wherein the second surface is interposed betweenthe reflective polarizer and the first surface.
 13. The head-mounteddisplay defined in claim 12, wherein the reflective polarizer has afirst pass axis and wherein the linear polarizer has a second pass axisthat is orthogonal to the first pass axis.
 14. The head-mounted displaydefined in claim 12, wherein the linear polarizer is a first linearpolarizer and wherein the lens module further comprises: a second linearpolarizer, wherein the reflective polarizer is interposed between thelens element and the second linear polarizer.
 15. The head-mounteddisplay defined in claim 14, wherein the lens module further comprises:a first layer of optically clear adhesive that is interposed between theretarder coating and the reflective polarizer; and a second layer ofoptically clear adhesive that is interposed between the reflectivepolarizer and the linear polarizer.
 16. The head-mounted display definedin claim 11, wherein the retarder coating is formed on the secondsurface.
 17. The head-mounted display defined in claim 11, wherein theretarder coating is formed on the first surface.
 18. The head-mounteddisplay defined in claim 17, wherein the retarder coating is a firstretarder coating and wherein the lens module further comprises: a secondretarder coating that is formed on the second surface.
 19. Thehead-mounted display defined in claim 11, wherein the retarder coatinghas a thickness that varies by less than 10% across the retardercoating.
 20. The head-mounted display defined in claim 11, wherein theretarder coating is a second quarter wave plate.
 21. The head-mounteddisplay defined in claim 11, wherein the retarder coating is areflective polarizer and retarder layer that is formed on the secondsurface.
 22. A head-mounted display configured to display imagesviewable by a user, comprising: an array of pixels configured to producethe images; a linear polarizer through which light associated with theimages passes; a quarter wave plate that receives the light from thelinear polarizer; and a lens module that receives the light from thequarter wave plate, wherein the lens module comprises: a lens elementhaving first and second opposing sides; a partially reflective mirror onthe first side of the lens element; and a cholesteric liquid crystallayer on the second side of the lens element.
 23. The head-mounteddisplay defined in claim 22, wherein the cholesteric liquid crystallayer forms a reflective polarizer and retarder layer.
 24. Thehead-mounted display defined in claim 22, wherein the cholesteric liquidcrystal layer has a thickness that varies by less than 10% across thecholesteric liquid crystal layer.
 25. The head-mounted display definedin claim 22, wherein the lens element has a first surface on the firstside and a second surface on the second side, wherein the partiallyreflective mirror is formed on the first surface, and wherein thecholesteric liquid crystal layer is formed on the second surface. 26.The head-mounted display defined in claim 25, wherein the first surfaceis an aspheric convex surface and wherein the second surface is anaspheric concave surface.
 27. The head-mounted display defined in claim22, wherein the partially reflective mirror is interposed between thelens element and the quarter wave plate.
 28. The head-mounted displaydefined in claim 22, wherein the partially reflective mirror transmits50% and reflects 50% of incident light.
 29. The head-mounted displaydefined in claim 22, wherein the linear polarizer is a first linearpolarizer and wherein the lens module further comprises: a second linearpolarizer, wherein the cholesteric liquid crystal layer is interposedbetween the lens element and the second linear polarizer.